Synchro-resolver systems or devices are typically employed to sense and control the position of rotating shafts accurately. In older prior art synchro-resolver systems, a motor is employed to drive a synchro at a particular rate or rotational velocity. The synchro produces a control signal which is fed into a resolver. The output of the resolver is then fed to a demodulator which produces an error signal that is input to an amplifier which drives the motor to be controlled.
The disadvantages of the previous methods are the cost, weight and corresponding electrical and mechanical hardware requirements for the additional drive motor and synchro. In addition, conventional resolver shaft position controlling circuits cannot be employed at very low speeds because the resolver must be excited by sine and cosine waveforms of fairly high frequency in order to function properly.
There have been attempts to overcome the limitations of conventional electro-mechanical synchros with the substitution of analog circuit synchro signal generators. An analog emulation of this synchro signal is undesirable, however, due to the instabilities of signals produced in analog circuitry.
Other prior art resolver excitation circuits have included digital circuit elements. For example, U.S. Pat. No. 4,204,257 discloses a resolver position measuring device which includes a digital counter, a pair of read only memory means for providing sine and cosine sequences and a pair of digital to analog converters to generate sine and cosine analog waveforms for resolver excitation. Circuits such as this do not solve the problem of poor resolver low frequency response.